A Brewer of Odrorir is charging Ironhide Brute, and killing him by charge attack (so dwarf now have privilege to Slay Movement) But Ironhide Brute was counterattacking this same time, hit the dwarf with 5,5 (critical attack) and knock him down because of Ironhide Brutes ability Critical Knock Down

My question:
1)What about Slay Movement in this situation?? All that effects/facts (in this situation Slay Movement and Knock Down) happen this same time immediatelly , and one of the effects potentially making the second effect impossible to make. It`s interrupted and simply lost?? Or maybe charging dwarf should stand up (spending 1 AcT) and do his slay move as usual? Or maybe dwarf was knocked down so he just lost his slay movement privilege, but still have to stand up if he has AcT?

I'd say that the activation of the brewer ends when he's knocked down. At page 98 there is a specific rule stating that "[knocked down models] must spend an action token to stand up during its next standard activation".
But if the warlord was a warsmith the issue wouldn't even present itself, since Clan sub-faction ability prevents models from being knocked down or involuntarily moved .

In this specific situation Brewer was supported by Runegate Keeper, so no way, brewer is knocked down for sure.
But actually Brewer has 3 AcT after succesfull charge. Should we treat him as "unconsious" during that whole round??

In other words: Does knock down effect end activation of the knocked down model??

I see what you mean, but the rules are a bit controversial. As I said the second paragraph concerning knocked down individual models explains that the action of standing up must happen in the next standard activation. However the successive note states that a model may stand up at any time during its standard activation. This might look a bit contradictory but, literally interpreted, the rules seem to point out that you can stand up at any time during your next standard activation. Maybe this was intended to represent the time a model needs in order to regain its senses.
Also if the Brewer can stand up in the same activation I'd say he cannot perform slay movement, but this is just common sense and therefore of little value .

I hope some moderator has the answer concerning whether if a model ends its activation when is knocked down or not as you pointed out.

Normally a model gets knock down when its not the active model, so i think thatswhy the rules say about next activation. To prevent a model standing up in the opponents turn when it has actions left. The rare case of a counterattacking model with critical knockdown is not found often...
But its still covered by the note. When you fall, just get up. But because of the interruption i expect the loss of the slay movement. Lost momentum. Just by common sense.

Indeed this is a quite tricky case, which might need a definitive word from a dev.

So far here are just a few notes from my personal point of view:
- The counter attack is simultaneous with the initial charge action, not with the slay movement.
- Hence the Brewer gets interrupted before he can even start his slay movement, which would have been an expansion of his charge action.
- Being knocked down prevents a model from doing anything before it gets up again.
- So my conclusion would be that the Brewer is knocked down before the actual slay movement, which he then loses because his action ends when getting knocked down and he needs to spend ACT to stand up - so he needs to make another action and cannot continue with slay movement afterwards. However, he would still have all the ACTs not spend on charging and standing up, so his activation would not be over.

- The counter attack is simultaneous with the initial charge action, not with the slay movement.
- Hence the Brewer gets interrupted before he can even start his slay movement, which would have been an expansion of his charge action.
- Being knocked down prevents a model from doing anything before it gets up again.
- So my conclusion would be that the Brewer is knocked down before the actual slay movement, which he then loses because his action ends when getting knocked down and he needs to spend ACT to stand up - so he needs to make another action and cannot continue with slay movement afterwards. However, he would still have all the ACTs not spend on charging and standing up, so his activation would not be over.

Additionally please note, knockdown does not end its activation. The rules do not state that; they only say t that at the start of the next round the model MUST stand up as the first action. Nothing precludes it standing up before that time, such as immediately after being knocked down.